Previously defined Antrim Shale is here raised in rank to Antrim Group and divided into three formations (ascending): Antrim (revised), Ellsworth (revised) , and Sanilac (new) Formations. The Sanilac consists of black shales formerly included within the upper part of the Ellsworth Shale in western MI and upper black shales within the Antrim Shale of eastern MI. Ellsworth Formation (revised) consists of gray shales below the upper black shales of the former Ellsworth in western MI and the gray shales of the "Upper" or "Light" Antrim of eastern MI. The Antrim Formation [of this report] is composed of black and gray shales below the Ellsworth and includes the previously known Traverse Formation. Antrim Formation is subdivided into (ascending) Mud Lake Gray Shale Member (Traverse Formation), Charlton Black Shale Member (lower black shale), Crapo Creek Gray Shale Member (middle gray shale), and Chester Black Shale Member (upper black shale). [Usage of the Antrim in this proposal is in violation of the NACSN, 1983, Art. 19(g), and is not recommended.]

Source: GNU records (USGS DDS-6; Reston GNULEX).